The present invention relates to data transmission, and in particular, to the transmission of data between digital processing elements operating at differing voltage ranges.
In many cases, it is important for one digital processing element, referred to as a xe2x80x9csource element,xe2x80x9d to provide data to another, referred to as the xe2x80x9ctarget element,xe2x80x9d for further processing.
One difficulty encountered in transmitting data in this manner is that any noise in the output signal of the source element is passed on to the target element. This can result in spurious errors in processing carried out by the target element.
Another difficulty encountered in transmitting data in this manner is that the signal voltage levels of a source element may not be those expected by the target element. This can occur when, for example, the source and target elements do not share a common power supply or grounding system.
One approach to transmitting data between a source and a target element is to transmit a differential signal. However, a difficulty in doing so is that the common mode range, which is the limited voltage range within which the signals must be kept, is often only 2V, with the upper limit at the most positive voltage (VCC). For PECL (pseudo emitter coupled logic) or LVDS (low voltage differential signals) systems having power supplies, variations in the supply voltage, even when well within specifications, may cause the common mode range voltage (VCMR) to exceed limits. A voltage drop on ground wires between the systems, caused by return current of other interfaces, electrostatic discharge, or ground currents of any source and frequency, may often also cause the common mode range voltage to exceed these limits. Although an excursion beyond the VCMR limits will cause permanent damage to a device in only extreme cases, such excursions are very likely to cause data errors.
In some cases, there may be a missing ground connection between the source and the target. This can result in particularly large potential differences between the source and the target.
In extreme cases, with 220V mains and a grounding error in both systems, the peak-to-peak voltage between the systems may exceed 300V.
In accordance with the invention, there is provided a method for coupling a digital signal generated by a digital processing unit by high-pass filtering the differential signal, thereby generating a filtered differential signal. The filtered differential signal is then provided to an input of a differential amplifier, the output of which is fed back to the input. This output can then be provided to, for example, a densecond digital processing unit, or to a transmission cable.
In one practice of the invention, the high-pass filtering of the differential signal is achieved by applying a first voltage to a first capacitor and applying a second voltage to a second capacitor. The difference between this first and second voltage defines the differential signal. A resistive shunt is then provided between the first and second capacitors.
The output of the differential amplifier can be fed back to the input by providing a feedback resistor between the output and the input. This can be achieved by providing a first feedback resistor between a positive output of the differential amplifier and a positive input of the differential amplifier and providing a second feedback resistor between a negative output of the differential amplifier and a negative input of the differential amplifier.
The invention also includes an interface circuit for coupling a differential signal generated by a digital processing unit. Such an interface circuit includes a high-pass filter having a high-pass-filter input for receiving the differential signal and a high-pass-filter output. The high-pass filter is coupled to a differential amplifier having a differential-amplifier input in electrical communication with the high-pass-filter output. An output of the differential-amplifier is coupled to a resistive feedback-network. This resistive feedback-network provides electrical communication between the differential-amplifier output and the differential-amplifier input.
These and other features and advantages of the invention will be apparent from the following description and the accompanying drawings in which: